Improved clinical outcome following liver transplant in patients with ethylmalonic encephalopathy

Red Flags in Primary Mitochondrial Diseases: What Should We Recognize?

Primary mitochondrial diseases (PMDs) are complex group of metabolic disorders caused by genetically determined impairment of the mitochondrial oxidative phosphorylation (OXPHOS). The unique features of mitochondrial genetics and the pivotal role of mitochondria in cell biology explain the phenotypical heterogeneity of primary mitochondrial diseases and the resulting diagnostic challenges that follow. Some peculiar features ("red flags") may indicate a primary mitochondrial disease, helping the physician to orient in this diagnostic maze. In this narrative review, we aimed to outline the features of the most common mitochondrial red flags offering a general overview on the topic that could help physicians to untangle mitochondrial medicine complexity.

Ethylmalonic Encephalopathy: a literature review and two new cases of mild phenotype

BACKGROUND

Ethylmalonic encephalopathy (EE) is a rare intoxication-type metabolic disorder with multisystem involvement. It is caused by mutations in ETHE1, which encodes the ETHE1 enzyme in the mitochondrial matrix that plays a key role in hydrogen sulfide (H2S) detoxification acting as a sulphur dioxygenase.

RESULTS

This review focuses on the clinical, metabolic, genetic and neuroradiological features of 70 reported cases, including two new cases. The common manifestations of EE are psychomotor regression, hypotonia, developmental delay, petechia, pyramidal signs, chronic diarrhoea, orthostatic acrocyanosis and failure to thrive, respectively. A significant difference was found in EMA and C4 levels (p=0.003, p=0.0236) between classical and mild phenotypes. Urinary EMA, C4 and C5 levels were found to exhibit normal values in milder cases during attack-free periods. The most common ETHE1 gene homozygous state mutations were (p.R163Q) (c.488G>A), exon 4 deletion, (p.R163W)(c.487C>T), (p.Glu44ValfsTer62)(c.131_132delAG) and (p.M1I)(c.3G>T) mutations, respectively. Fifty-two patients underwent cranial MRI. Basal ganglia signal alterations were detected in 42 cases. Of the 70 cases, eight had a mild phenotype and slow neurological progression with low levels of ethylmalonic acid (EMA) and C4 acylcarnitine. The current age of alive patients in the published articles with mild phenotype was significantly higher than the classical phenotype. (p=0.002). Reducing the accumulation and inducing detoxification of sulfide is the main long-term treatment strategy for EE, including metronidazole, N-acetylcysteine (NAC), dietary modification, liver transplantation and continuous renal replacement therapy (CRRT).

CONCLUSION

Measuring EMA and C4 acylcarnitine during metabolic attacks is critical to diagnosing EE, allowing for early treatment initiation to prevent further encephalopathic crises. Experience with liver transplantation, diet and CRRT, is currently limited. An early multidisciplinary approach with combination therapies is vital to prevent irreversible neurological damage.

Clinical spectrum and genetic causes of mitochondrial hepatopathy phenotype in children

BACKGROUND

Alterations in both mitochondrial DNA (mtDNA) and nuclear DNA genes affect mitochondria function, causing a range of liver-based conditions termed mitochondrial hepatopathies (MH), which are subcategorized as mtDNA depletion, RNA translation, mtDNA deletion, and enzymatic disorders. We aim to enhance the understanding of pathogenesis and natural history of MH.

METHODS

We analyzed data from patients with MH phenotypes to identify genetic causes, characterize the spectrum of clinical presentation, and determine outcomes.

RESULTS

Three enrollment phenotypes, that is, acute liver failure (ALF, n = 37), chronic liver disease (Chronic, n = 40), and post-liver transplant (n = 9), were analyzed. Patients with ALF were younger [median 0.8 y (range, 0.0, 9.4) vs 3.4 y (0.2, 18.6), p < 0.001] with fewer neurodevelopmental delays (40.0% vs 81.3%, p < 0.001) versus Chronic. Comprehensive testing was performed more often in Chronic than ALF (90.0% vs 43.2%); however, etiology was identified more often in ALF (81.3% vs 61.1%) with mtDNA depletion being most common (ALF: 77% vs Chronic: 41%). Of the sequenced cohort (n = 60), 63% had an identified mitochondrial disorder. Cluster analysis identified a subset without an underlying genetic etiology, despite comprehensive testing. Liver transplant-free survival was 40% at 2 years (ALF vs Chronic, 16% vs 65%, p < 0.001). Eighteen (21%) underwent transplantation. With 33 patient-years of follow-up after the transplant, 3 deaths were reported.

CONCLUSIONS

Differences between ALF and Chronic MH phenotypes included age at diagnosis, systemic involvement, transplant-free survival, and genetic etiology, underscoring the need for ultra-rapid sequencing in the appropriate clinical setting. Cluster analysis revealed a group meeting enrollment criteria but without an identified genetic or enzymatic diagnosis, highlighting the need to identify other etiologies.

An atypically mild case of ethylmalonic encephalopathy with pathogenic ETHE1 variant

Ethylmalonic encephalopathy (EE) is a rare, severe, autosomal recessive condition caused by pathogenic variants in ETHE1 leading to progressive encephalopathy, hypotonia evolving to dystonia, petechiae, orthostatic acrocyanosis, diarrhea, and elevated ethylmalonic acid in urine. In this case report, we describe a patient with only mild speech and gross motor delays, subtle biochemical abnormalities, and normal brain imaging found to be homozygous for a pathogenic ETHE1 variant (c.586G>A) via whole exome sequencing. This case highlights the clinical heterogeneity of ETHE1 mutations and the utility of whole-exome sequencing in diagnosing mild cases of EE.

Depleted uranium causes renal mitochondrial dysfunction through the ETHE1/Nrf2 pathway

The kidney is the main organ affected by acute depleted uranium (DU) toxicity. The mechanism of nephrotoxicity induced by DU is complex and needs to be further explored. This study aimed to elucidate the function of mitochondrial dysfunction in nephrotoxicity generated by DU and confirm the latent mechanism. We verified that DU (2.5-10 mg/kg) caused mitochondrial dysfunction in male rat kidneys and decreased ATP content and the mitochondrial membrane potential. In addition, melatonin (20 mg/kg), as an antioxidant, alleviated DU-induced oxidative stress and mitochondrial dysfunction in male rats, further reducing kidney damage caused by DU. These results indicate that mitochondrial dysfunction plays a vital role in DU nephrotoxicity. When ethylmalonic encephalopathy 1 (ETHE1) was knocked down, DU-induced oxidative stress and mitochondrial dysfunction were increased, and renal injury was aggravated. When exogenous ETHE1 protein was applied to renal cells, the opposite changes were observed. We also found that ETHE1 knockdown increased the expression of NF-E2-related factor 2 (Nrf2), a vital oxidative stress regulator, and its downstream molecules heme oxygenase-1 (HO-1) and NADPH quinone oxidoreductase 1 (NQO1). Nrf2 knockout also aggravated DU-induced oxidative stress, mitochondrial dysfunction, and kidney damage. In conclusion, DU causes oxidative stress and antioxidant defense imbalance in renal cells through the ETHE1/Nrf2 pathway, further causing mitochondrial dysfunction and ultimately leading to nephrotoxicity.

Human ultrarare genetic disorders of sulfur metabolism demonstrate redundancies in H2S homeostasis

Regulation of H₂S homeostasis in humans is poorly understood. Therefore, we assessed the importance of individual enzymes in synthesis and catabolism of H₂S by studying patients with respective genetic defects. We analyzed sulfur compounds (including bioavailable sulfide) in 37 untreated or insufficiently treated patients with seven ultrarare enzyme deficiencies and compared them to 63 controls. Surprisingly, we observed that patients with severe deficiency in cystathionine β-synthase (CBS) or cystathionine γ-lyase (CSE) - the enzymes primarily responsible for H₂S synthesis - exhibited increased and normal levels of bioavailable sulfide, respectively. However, an approximately 21-fold increase of urinary homolanthionine in CBS deficiency strongly suggests that lacking CBS activity is compensated for by an increase in CSE-dependent H₂S synthesis from accumulating homocysteine, which suggests a control of H₂S homeostasis in vivo. In deficiency of sulfide:quinone oxidoreductase - the first enzyme in mitochondrial H₂S oxidation - we found normal H₂S concentrations in a symptomatic patient and his asymptomatic sibling, and elevated levels in an asymptomatic sibling, challenging the requirement for this enzyme in catabolizing H₂S under physiological conditions. Patients with ethylmalonic encephalopathy and sulfite oxidase/molybdenum cofactor deficiencies exhibited massive accumulation of thiosulfate and sulfite with formation of large amounts of S-sulfocysteine and S-sulfohomocysteine, increased renal losses of sulfur compounds and concomitant strong reduction in plasma total cysteine. Our results demonstrate the value of a comprehensive assessment of sulfur compounds in severe disorders of homocysteine/cysteine metabolism and provide evidence for redundancy and compensatory mechanisms in the maintenance of H₂S homeostasis.

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Cystathionine γ-lyase can compensate for decreased H₂S synthesis in cystathionine β-synthase deficiency.

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Sulfide:quinone oxidoreductase deficiency is compatible with normal H₂S plasma levels under non-stressed conditions.

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Persulfide dioxygenase deficiency (ethylmalonic encephalopathy) causes the largest accumulation of H₂S among disorders of sulfur metabolism.

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Excess sulfite forms S-sulfocysteine and S-sulfohomocysteine, and interferes with vitamin B₆ metabolism.

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S-sulfocysteine correlates directly with sulfite and is a stable biomarker of sulfite accumulation.

Gene Therapy for Mitochondrial Diseases: Current Status and Future Perspective

Mitochondrial diseases (MDs) are a group of severe genetic disorders caused by mutations in the nuclear or mitochondrial genome encoding proteins involved in the oxidative phosphorylation (OXPHOS) system. MDs have a wide range of symptoms, ranging from organ-specific to multisystemic dysfunctions, with different clinical outcomes. The lack of natural history information, the limits of currently available preclinical models, and the wide range of phenotypic presentations seen in MD patients have all hampered the development of effective therapies. The growing number of pre-clinical and clinical trials over the last decade has shown that gene therapy is a viable precision medicine option for treating MD. However, several obstacles must be overcome, including vector design, targeted tissue tropism and efficient delivery, transgene expression, and immunotoxicity. This manuscript offers a comprehensive overview of the state of the art of gene therapy in MD, addressing the main challenges, the most feasible solutions, and the future perspectives of the field.

AAV-vector based gene therapy for mitochondrial disease: progress and future perspectives

Mitochondrial diseases are a group of rare, heterogeneous diseases caused by gene mutations in both nuclear and mitochondrial genomes that result in defects in mitochondrial function. They are responsible for significant morbidity and mortality as they affect multiple organ systems and particularly those with high energy-utilizing tissues, such as the nervous system, skeletal muscle, and cardiac muscle. Virtually no effective treatments exist for these patients, despite the urgent need. As the majority of these conditions are monogenic and caused by mutations in nuclear genes, gene replacement is a highly attractive therapeutic strategy. Adeno-associated virus (AAV) is a well-characterized gene replacement vector, and its safety profile and ability to transduce quiescent cells nominates it as a potential gene therapy vehicle for several mitochondrial diseases. Indeed, AAV vector-based gene replacement is currently being explored in clinical trials for one mitochondrial disease (Leber hereditary optic neuropathy) and preclinical studies have been published investigating this strategy in other mitochondrial diseases. This review summarizes the preclinical findings of AAV vector-based gene replacement therapy for mitochondrial diseases including Leigh syndrome, Barth syndrome, ethylmalonic encephalopathy, and others.

Hydrogen sulphide in liver glucose/lipid metabolism and non-alcoholic fatty liver disease

BACKGROUND

For a long time, hydrogen sulphide (H₂ S) was considered only as a toxic gas, inhibiting mitochondrial respiration at the level of cytochrome c oxidase, and an environmental pollutant. Nowadays, H₂ S is recognized as the third mammalian gasotransmitter, playing an important role in inflammation, septic shock, ischaemia reperfusion events, cardiovascular disease and more recently in liver physiology and chronic liver diseases such as non-alcoholic fatty liver disease (NAFLD).

METHODS

This narrative review is based on literature search using PubMed.

RESULTS

From a bioenergetic perspective, H₂ S is a very unique molecule, serving as a mitochondrial poison at high concentrations or as an inorganic mitochondrial substrate at low concentrations. By using transgenic animal models to specifically modulate liver H₂ S biosynthesis or exogenous compounds that release H₂ S, several studies demonstrated that H₂ S is a key player in liver glucose and lipid metabolism. Liver H₂ S content and biosynthesis were also altered in NAFLD animal models with the in vivo administration of H₂ S-releasing molecules preventing the further escalation into non-alcoholic-steatohepatitis. Liver steady-state levels of H₂ S, and hence its cell signalling properties, are controlled by a tight balance between its biosynthesis, mainly through the transsulphuration pathway, and its mitochondrial oxidation via the sulphide oxidizing unit. However, studies investigating mitochondrial H₂ S oxidation in liver dysfunction still remain scarce.

CONCLUSIONS

Since H₂ S emerges as a key regulator of liver metabolism and metabolic flexibility, further understanding the physiological relevance of mitochondrial H₂ S oxidation in liver energy homeostasis and its potential implication in chronic liver diseases are of great interest.

Acute Strokelike Presentation and Long-term Evolution of Diffusion Restriction Pattern in Ethylmalonic Encephalopathy

Ethylmalonic encephalopathy is a rare autosomal recessive mitochondrial disorder caused by pathogenic biallelic variants in the ETHE1 gene. The phenotype of this disease has been attributed to deficiency in the mitochondrial sulfur dioxygenase leading to many downstream effects. Ethylmalonic encephalopathy classically presents with developmental regression, petechiae, acrocyanosis, and chronic diarrhea. The neurologic phenotype includes hypotonia, spastic diplegia, ataxia, and developmental delay. As more patients with this condition are described, the neurologic phenotype continues to expand. Although strokelike episodes or metabolic strokes have been studied in other mitochondrial disorders, they have not been thoroughly reported in this disorder. Herein, we describe 3 patients with ethylmalonic encephalopathy who presented clinically with strokelike episodes and strokelike abnormalities on brain magnetic resonance imaging in the setting of acute illness, and the long-term sequelae with evolution into cystic changes in one of these subjects.

Comparison of Untargeted Metabolomic Profiling vs Traditional Metabolic Screening to Identify Inborn Errors of Metabolism

IMPORTANCE

Recent advances in newborn screening (NBS) have improved the diagnosis of inborn errors of metabolism (IEMs); however, many potentially treatable IEMs are not included on NBS panels, nor are they covered in standard, first-line biochemical testing.

OBJECTIVE

To examine the utility of untargeted metabolomics as a primary screening tool for IEMs by comparing the diagnostic rate of clinical metabolomics with the recommended traditional metabolic screening approach.

DESIGN, SETTING, AND PARTICIPANTS

This cross-sectional study compares data from 4464 clinical samples received from 1483 unrelated families referred for trio testing of plasma amino acids, plasma acylcarnitine profiling, and urine organic acids (June 2014 to October 2018) and 2000 consecutive plasma samples from 1807 unrelated families (July 2014 to February 2019) received for clinical metabolomic screening at a College of American Pathologists and Clinical Laboratory Improvement Amendments-certified biochemical genetics laboratory. Data analysis was performed from September 2019 to August 2020.

EXPOSURES

Metabolic and molecular tests performed at a genetic testing reference laboratory in the US and available clinical information for each patient were assessed to determine diagnostic rate.

MAIN OUTCOMES AND MEASURES

The diagnostic rate of traditional metabolic screening compared with clinical metabolomic profiling was assessed in the context of expanded NBS.

RESULTS

Of 1483 cases screened by the traditional approach, 912 patients (61.5%) were male and 1465 (98.8%) were pediatric (mean [SD] age, 4.1 [6.0] years; range, 0-65 years). A total of 19 families were identified with IEMs, resulting in a 1.3% diagnostic rate. A total of 14 IEMs were detected, including 3 conditions not included in the Recommended Uniform Screening Panel for NBS. Of the 1807 unrelated families undergoing plasma metabolomic profiling, 1059 patients (58.6%) were male, and 1665 (92.1%) were pediatric (mean [SD] age, 8.1 [10.4] years; range, 0-80 years). Screening identified 128 unique cases with IEMs, giving an overall diagnostic rate of 7.1%. In total, 70 different metabolic conditions were identified, including 49 conditions not presently included on the Recommended Uniform Screening Panel for NBS.

CONCLUSIONS AND RELEVANCE

These findings suggest that untargeted metabolomics provided a 6-fold higher diagnostic yield compared with the conventional screening approach and identified a broader spectrum of IEMs. Notably, with the expansion of NBS programs, traditional metabolic testing approaches identify few disorders beyond those covered on the NBS. These data support the capability of clinical untargeted metabolomics in screening for IEMs and suggest that broader screening approaches should be considered in the initial evaluation for metabolic disorders.

Ethylmalonic encephalopathy and liver transplantation: long-term outcome of the first treated patient

BACKGROUND

Ethylmalonic encephalopathy (EE) is a severe intoxication-type metabolic disorder with multisystem clinical features and leading to early death. In 2014, based on the promising results obtained by liver-targeted gene therapy in Ethe1-/- mouse model, we successfully attempted liver transplantation in a 9-month-old EE girl. Here we report her long-term follow-up, lasting over 6 years, with a comprehensive evaluation of clinical, instrumental and biochemical assessments.

RESULTS

Neurological signs initially reverted, with a clinical stabilization during the entire follow-up course. Accordingly, gross motor functions improved and then stabilized. Psychomotor evaluations documented an increasing communicative intent, the acquisition of new social skills and the capability to carry out simple orders. Neurophysiological assessments, which included EEG, VEP/ERG and BAEPs, remained unchanged. Brain MRI also stabilized, showing no further lesions and cerebral atrophy improvement. Compared to pre-transplant assessments, urinary ethylmalonic acid strikingly reduced, and plasma thiosulphate fully normalized. The child maintained good clinical conditions and never experienced metabolic crises nor epileptic seizures.

CONCLUSIONS

The long-term follow-up of the first EE transplanted patient demonstrates that liver transplantation stabilizes, or even improves, disease course, therefore representing a potentially elective option especially in early-diagnosed patients, such as those detected by newborn screening, before irreversible neurological damage occurs.

Ethylmalonic acid impairs bioenergetics by disturbing succinate and glutamate oxidation and induces mitochondrial permeability transition pore opening in rat cerebellum

Tissue accumulation and high urinary excretion of ethylmalonic acid (EMA) are found in ethylmalonic encephalopathy (EE), an inherited disorder associated with cerebral and cerebellar atrophy whose pathogenesis is poorly established. The in vitro and in vivo effects of EMA on bioenergetics and redox homeostasis were investigated in rat cerebellum. For the in vitro studies, cerebellum preparations were exposed to EMA, whereas intracerebellar injection of EMA was used for the in vivo evaluation. EMA reduced state 3 and uncoupled respiration in vitro in succinate-, glutamate-, and malate-supported mitochondria, whereas decreased state 4 respiration was observed using glutamate and malate. Furthermore, mitochondria permeabilization and succinate supplementation diminished the decrease in state 3 with succinate. EMA also inhibited the activity of KGDH, an enzyme necessary for glutamate oxidation, in a mixed manner and augmented mitochondrial efflux of α-ketoglutarate. ATP levels were markedly reduced by EMA, reflecting a severe bioenergetic disruption. Docking simulations also indicated interactions between EMA and KGDH and a competition with glutamate and succinate for their mitochondrial transporters. In vitro findings also showed that EMA decreased mitochondrial membrane potential and Ca²⁺ retention capacity, and induced swelling in the presence of Ca²⁺ , which were prevented by cyclosporine A and ADP and ruthenium red, indicating mitochondrial permeability transition (MPT). Moreover, EMA, at high concentrations, mildly increased ROS levels and altered antioxidant defenses in vitro and in vivo. Our data indicate that EMA-induced impairment of glutamate and succinate oxidation and MPT may contribute to the pathogenesis of the cerebellum abnormalities in EE.

Treatable inherited metabolic disorders causing intellectual disability: 2021 review and digital app

BACKGROUND

The Treatable ID App was created in 2012 as digital tool to improve early recognition and intervention for treatable inherited metabolic disorders (IMDs) presenting with global developmental delay and intellectual disability (collectively 'treatable IDs'). Our aim is to update the 2012 review on treatable IDs and App to capture the advances made in the identification of new IMDs along with increased pathophysiological insights catalyzing therapeutic development and implementation.

METHODS

Two independent reviewers queried PubMed, OMIM and Orphanet databases to reassess all previously included disorders and therapies and to identify all reports on Treatable IDs published between 2012 and 2021. These were included if listed in the International Classification of IMDs (ICIMD) and presenting with ID as a major feature, and if published evidence for a therapeutic intervention improving ID primary and/or secondary outcomes is available. Data on clinical symptoms, diagnostic testing, treatment strategies, effects on outcomes, and evidence levels were extracted and evaluated by the reviewers and external experts. The generated knowledge was translated into a diagnostic algorithm and updated version of the App with novel features.

RESULTS

Our review identified 116 treatable IDs (139 genes), of which 44 newly identified, belonging to 17 ICIMD categories. The most frequent therapeutic interventions were nutritional, pharmacological and vitamin and trace element supplementation. Evidence level varied from 1 to 3 (trials, cohort studies, case-control studies) for 19% and 4-5 (case-report, expert opinion) for 81% of treatments. Reported effects included improvement of clinical deterioration in 62%, neurological manifestations in 47% and development in 37%.

CONCLUSION

The number of treatable IDs identified by our literature review increased by more than one-third in eight years. Although there has been much attention to gene-based and enzyme replacement therapy, the majority of effective treatments are nutritional, which are relatively affordable, widely available and (often) surprisingly effective. We present a diagnostic algorithm (adjustable to local resources and expertise) and the updated App to facilitate a swift and accurate workup, prioritizing treatable IDs. Our digital tool is freely available as Native and Web App (www.treatable-id.org) with several novel features. Our Treatable ID endeavor contributes to the Treatabolome and International Rare Diseases Research Consortium goals, enabling clinicians to deliver rapid evidence-based interventions to our rare disease patients.

Inherited disorders of sulfur amino acid metabolism: recent advances in therapy

PURPOSE OF REVIEW

Metabolism of sulfur amino acids (SAA) provides compounds important for many cellular functions. Inherited disorders of SAA metabolism are typically severe multisystemic diseases affecting brain, liver, connective tissue, or vasculature. The review summarizes the present therapeutic approaches and advances in identifying novel treatment targets, and provides an overview of new therapies.

RECENT FINDINGS

Current treatments of genetic disorders of SAA metabolism are primarily based on modulation of affected pathways by dietary measures and provision of lacking products or scavenging of toxic molecules. Recent studies identified additional therapeutic targets distant from the primary defects and explored ideas envisioning novel treatments, such as chaperone and gene therapy. Recombinant protein production and engineering resulted in development and clinical testing of enzyme therapies for cystathionine β-synthase deficiency, the most common inborn error of SAA metabolism.

SUMMARY

Complex regulation of pathways involved in SAA metabolism and cellular consequences of genetic defects in SAA metabolism are only partially understood. There is a pressing need to increase substantially our knowledge of the disease mechanisms to develop more effective therapies for patients suffering from these rare disorders.

Current progress in the therapeutic options for mitochondrial disorders

Mitochondrial disorders manifest enormous genetic and clinical heterogeneity - they can appear at any age, present with various phenotypes affecting any organ, and display any mode of inheritance. What mitochondrial diseases do have in common, is impairment of respiratory chain activity, which is responsible for more than 90% of energy production within cells. While diagnostics of mitochondrial disorders has been accelerated by introducing Next-Generation Sequencing techniques in recent years, the treatment options are still very limited. For many patients only a supportive or symptomatic therapy is available at the moment. However, decades of basic and preclinical research have uncovered potential target points and numerous compounds or interventions are now subjects of clinical trials. In this review, we focus on current and emerging therapeutic approaches towards the treatment of mitochondrial disorders. We focus on small compounds, metabolic interference, such as endurance training or ketogenic diet and also on genomic approaches.

Therapeutic Approaches to Treat Mitochondrial Diseases: "One-Size-Fits-All" and "Precision Medicine" Strategies

Primary mitochondrial diseases (PMD) refer to a group of severe, often inherited genetic conditions due to mutations in the mitochondrial genome or in the nuclear genes encoding for proteins involved in oxidative phosphorylation (OXPHOS). The mutations hamper the last step of aerobic metabolism, affecting the primary source of cellular ATP synthesis. Mitochondrial diseases are characterized by extremely heterogeneous symptoms, ranging from organ-specific to multisystemic dysfunction with different clinical courses. The limited information of the natural history, the limitations of currently available preclinical models, coupled with the large variability of phenotypical presentations of PMD patients, have strongly penalized the development of effective therapies. However, new therapeutic strategies have been emerging, often with promising preclinical and clinical results. Here we review the state of the art on experimental treatments for mitochondrial diseases, presenting "one-size-fits-all" approaches and precision medicine strategies. Finally, we propose novel perspective therapeutic plans, either based on preclinical studies or currently used for other genetic or metabolic diseases that could be transferred to PMD.

Compromised therapeutic value of pediatric liver transplantation in ethylmalonic encephalopathy: A case report

BACKGROUND

Ethylmalonic encephalopathy (EE) is a rare autosomal recessive metabolic disorder caused by impaired mitochondrial sulfur dioxygenase. Due to poor therapeutic effect of current conventional treatments, progressive psychomotor regression and neurological impairment usually contribute to early death in the first decade of life. Liver transplantation (LT) is emerging as a novel therapeutic option for EE; however, worldwide experience is still limited.

CASE SUMMARY

An 18-mo-old patient with the diagnosis of EE received a living donor liver transplant in our institution, which, to our knowledge, is the first case in Asian-Pacific countries. During 20 mo of follow-up, the longitudinal metabolic evaluations revealed a wild fluctuation in urinary EMA levels, still far beyond the normal range. Urinary 2-methylsuccinic acid levels gradually restored to normal, whereas the concentrations of urinary isobutyrylglycine and plasma C4- and C5-acylcarnitines fluctuated markedly and still remained above the reference limits. Only mild amelioration of petechiae and ecchymosis was observed, and no dramatic reversion of chronic mucoid diarrhea and orthostatic acrocyanosis occurred. Although brain magnetic resonance imaging suggested a certain improvement in basal ganglia lesions, the patient still presented developmental delay and neurologic disability.

CONCLUSION

LT may bring about a partial but not complete cure to EE. Given its definite role in defending against the devastating natural progression of EE, LT should still be considered for patients with EE in the absence of other superior therapeutic options. Early establishment of diagnosis and initiation of conventional treatment pre-transplant, timely LT, and continuous administration of metabolism-correcting medications post-transplant may be helpful in minimizing the neurologic impairment and maximizing the therapeutic value of LT in EE.

Lessons Learned from Inherited Metabolic Disorders of Sulfur-Containing Amino Acids Metabolism

The metabolism of sulfur-containing amino acids (SAAs) requires an orchestrated interplay among several dozen enzymes and transporters, and an adequate dietary intake of methionine (Met), cysteine (Cys), and B vitamins. Known human genetic disorders are due to defects in Met demethylation, homocysteine (Hcy) remethylation, or cobalamin and folate metabolism, in Hcy transsulfuration, and Cys and hydrogen sulfide (H2S) catabolism. These disorders may manifest between the newborn period and late adulthood by a combination of neuropsychiatric abnormalities, thromboembolism, megaloblastic anemia, hepatopathy, myopathy, and bone and connective tissue abnormalities. Biochemical features include metabolite deficiencies (e.g. Met, S-adenosylmethionine (AdoMet), intermediates in 1-carbon metabolism, Cys, or glutathione) and/or their accumulation (e.g. S-adenosylhomocysteine, Hcy, H2S, or sulfite). Treatment should be started as early as possible and may include a low-protein/low-Met diet with Cys-enriched amino acid supplements, pharmacological doses of B vitamins, betaine to stimulate Hcy remethylation, the provision of N-acetylcysteine or AdoMet, or experimental approaches such as liver transplantation or enzyme replacement therapy. In several disorders, patients are exposed to long-term markedly elevated Met concentrations. Although these conditions may inform on Met toxicity, interpretation is difficult due to the presence of additional metabolic changes. Two disorders seem to exhibit Met-associated toxicity in the brain. An increased risk of demyelination in patients with Met adenosyltransferase I/III (MATI/III) deficiency due to biallelic mutations in the MATIA gene has been attributed to very high blood Met concentrations (typically >800 μmol/L) and possibly also to decreased liver AdoMet synthesis. An excessively high Met concentration in some patients with cystathionine β-synthase deficiency has been associated with encephalopathy and brain edema, and direct toxicity of Met has been postulated. In summary, studies in patients with various disorders of SAA metabolism showed complex metabolic changes with distant cellular consequences, most of which are not attributable to direct Met toxicity.

Clinical trials in mitochondrial disorders, an update

Mitochondrial disorders comprise a molecular and clinically diverse group of diseases that are associated with mitochondrial dysfunction leading to multi-organ disease. With recent advances in molecular technologies, the understanding of the pathomechanisms of a growing list of mitochondrial disorders has been greatly expanded. However, the therapeutic approaches for mitochondrial disorders have lagged behind with treatment options limited mainly to symptom specific therapies and supportive measures. There is an increasing number of clinical trials in mitochondrial disorders aiming for more specific and effective therapies. This review will cover different treatment modalities currently used in mitochondrial disorders, focusing on recent and ongoing clinical trials.

Riboflavin Deficiency-Implications for General Human Health and Inborn Errors of Metabolism

As an essential vitamin, the role of riboflavin in human diet and health is increasingly being highlighted. Insufficient dietary intake of riboflavin is often reported in nutritional surveys and population studies, even in non-developing countries with abundant sources of riboflavin-rich dietary products. A latent subclinical riboflavin deficiency can result in a significant clinical phenotype when combined with inborn genetic disturbances or environmental and physiological factors like infections, exercise, diet, aging and pregnancy. Riboflavin, and more importantly its derivatives, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), play a crucial role in essential cellular processes including mitochondrial energy metabolism, stress responses, vitamin and cofactor biogenesis, where they function as cofactors to ensure the catalytic activity and folding/stability of flavoenzymes. Numerous inborn errors of flavin metabolism and flavoenzyme function have been described, and supplementation with riboflavin has in many cases been shown to be lifesaving or to mitigate symptoms. This review discusses the environmental, physiological and genetic factors that affect cellular riboflavin status. We describe the crucial role of riboflavin for general human health, and the clear benefits of riboflavin treatment in patients with inborn errors of metabolism.

Novel Compound Heterozygous Variants of ETHE1 Causing Ethylmalonic Encephalopathy in a Chinese Patient: A Case Report

Ethylmalonic encephalopathy (EE) is a very rare autosomal recessive metabolic disorder that primarily affects children. Less than one hundred EE patients have been diagnosed worldwide. The clinical manifestations include chronic diarrhea, petechiae, orthostatic acrocyanosis, psychomotor delay and regression, seizures, and hypotonia. The ETHE1 gene has been shown to be associated with EE, and genetic sequencing provides concrete evidence for diagnosis. To date, only 37 variants of ETHE1 have been reported as disease-causing in EE patients. We identified two novel ETHE1 variants, i.e., c.595+1G>T at the canonical splice site and the missense variant c.586G>C (p. D196H), in a 3-year-old Chinese boy with EE. The patient had mild symptoms with only chronic diarrhea. The typical symptoms, including spontaneous petechiae, acrocyanosis, and hypotonia, were all absent. Herein, we report on the clinical, biochemical, and genetic findings of our patient and review the phenotypes and genotypes of all patients with EE caused by ETHE1 variants with available information. This study supports the early assessment and diagnosis of EE.